1. Field of the Invention
The present invention relates to an electric power steering device and, particularly, to an electric power steering device capable of detecting a failure of its motor current detection device.
2. Description of the Prior Art
An electric power steering device for vehicles detects steering torque generated in a steering shaft by the operation of a steering wheel and car speed, and drives a motor based on the detection signal to assist the steering force of the steering wheel. This electric power steering device is controlled by an electronic control unit. As for its control mechanism, the level of current to be supplied to the motor is calculated based on steering torque detected by a torque sensor and car speed detected by a car speed sensor.
That is, the electronic control unit controls the current to be supplied to the motor based on the steering torque and the car speed so that large steering assist force is supplied when the car speed is low, while steering torque is generated is zero or low and small steering assist force is supplied when the detected car speed is high, thereby making it possible to provide the optimum steering assist force corresponding to the running state of the vehicle.
Feed-back control is carried out so that the actual motor current through the motor becomes the motor current command value calculated based on the steering torque and the car speed. Therefore, this type of device comprises a motor current detection device for detecting an actual current through the motor.
If the above motor current detection device fails, an accurate motor current cannot be measured. As a result, motor current larger than the required current flows through the motor to supply excessive steering assist force, or that a required current does not flow through the motor, making it impossible to supply sufficient steering assist force.
Further, if the motor incorrectly rotates when the operation of the faulty motor current detection device is checked, the steering wheel linked to the motor shaft through the steering mechanism may incorrectly rotate, whereby an accident may occur.
As a solution to this problem, the applicant proposes a failure judging device for judging a failure of the motor current detection device based on a motor current estimated value when a low voltage is supplied to the above motor for a time much larger than the electric time constant of the motor and much smaller than the mechanical time constant of the motor and a motor current value detected by the motor current detection device (Japanese Laid Open Patent Publication H08-91239 (91239/1996)).
The above failure judging device judges a failure by supplying a low voltage to the motor for a limited time just after the start of the engine by turning on an ignition key, that is, for a time much larger than the electric time constant of the motor and much smaller than the mechanical time constant of the motor. This is necessary to prevent an unexpected accident which is caused by the abrupt turning of the steering wheel when the motor rotates just after the start of the engine.
However, although no failure occurs when the motor is new, after the motor is used for a certain period of time, an oxide film having electrical insulating properties forms on the contact surface between the commutator and brush of the motor and becomes thick as time elapses, thereby increasing electric resistance. As a result, unless a higher voltage is supplied to the motor, a current does not flow through the motor.
In this case, when a low voltage is supplied to the motor for a short time in order to judge a failure, a motor current is not detected or a small value is detected due to the oxide film formed on the contact surface between the commutator and the brush. Therefore, it may be misjudged that the motor current detection device has failed.
To solve this problem, the applicant proposes a failure judging device for judging a failure of the motor current detection device by setting a motor current command value for a time much larger than the electric time constant of the motor and much smaller than the mechanical time constant of the motor, changing a motor current control value based on the motor current command value as time elapses and comparing a motor current estimated value with a motor current detection value detected by the motor current detection device while increasing motor supply voltage from a low voltage as time elapses (Japanese laid Open Patent Publication 2003-237605 (237605/2003)).
However, in this failure judging device for detecting a failure of the motor current detection device, the timing of sampling a motor current is set just before the application of voltage to the motor is stopped.
Therefore, normal voltage is supplied to the motor, and the motor current detected value is normal and higher than the threshold value for judging a failure. If the motor supply voltage drops for some reason or other just before the motor supply voltage is stopped, the sampled motor current detection value falls below the threshold value for judging a failure. Therefore, even though the motor current detection device is normal, the failure judging device misjudges that the motor current detection device has failed.
The cause of drop down of the motor supply voltage may not be that an oxide film is formed on the contact surface between the commutator and brush of the motor. It is conceivable that the resistance of a wire harness between the motor and a control unit increases, the resistance is increased by the dislocation of contact between the commutator and brush of the motor, an oxide film is formed on the contact point of a relay when the motor relay is interposed between the motor and the battery, or foreign substance adheres to the contact point of the motor relay besides a case where the power supply voltage (battery voltage) drops for some reason or other. The present invention is aimed to solve the above problems.